A pressure-sensitive adhesive exhibiting excellent tack property at low temperatures and ensuring sufficiently high cohesive force and holding force is demanded in the field of pressure-sensitive adhesive tapes, particularly, a masking tape for sealing in the architectural field. The sealing operation includes the steps of 1) applying a masking tape to a site intended to be prevented from contamination by a sealing material, 2) coating a primer for sealing materials, 3) coating a sealing material, and 4) after drying or curing the sealing material, peeling off the masking tape.
As regards the masking tape for sealing, an acrylic pressure-sensitive adhesive is being studied, and the following acrylic pressure sensitive adhesives are known.
Patent Document 1 (Kokai (Japanese Unexamined Patent Publication) No. 6-346039) describes “a pressure-sensitive adhesive composition, which is an aqueous emulsion pressure-sensitive adhesive composition, comprising (A) a polymer obtained by the polymerization of monomers containing (1) 100 parts by weight of an acrylate monomer and (2) from 0.1 to 5 parts by weight of a carbonyl group-containing monomer, the polymer having a glass transition point of −75 to −45° C. and a storage modulus of 1×105 to 4×106 dyn/cm2; and (B) a polyfunctional hydrazide compound in an amount of 0.02 to 0.5 mol per mol of the carbonyl group in the carbonyl group-containing monomer of (2)”. In Patent Document 1, the alkyl group of the acrylate monomer is specified to have a carbon number of 9 or less.
Patent Document 2 (Kohyo (National Publication of Translated Version) No. 7-502560) describes “an acrylic pressure-sensitive adhesive, which substantially exhibits self-adhesive property, comprising:                a) from 10 to 50 wt % of a higher alkyl acrylate with the alkyl group having a carbon number of 12 to 26,        b) from 50 to 90 wt % of a lower alkyl acrylate with the alkyl group having a carbon number of 4 to 12,        c) at least one kind of a polar monomer copolymerizable with the higher alkyl acrylate and lower alkyl acrylate, and        d) a crosslinking agent in an amount sufficiently large to endow the adhesive with satisfactory cohesive strength for preventing substantial migration of the adhesive”. The crosslinking agents set forth includes a polyfunctional acrylate, a polyfunctional methacrylate, a chromophore-substituted halomethyl-S-triazine, benzophenone, acetophenone, silane, a monoethylenically unsaturated aromatic ketone, and a combination thereof. Photo-crosslinking is caused in the melt-coating type pressure-sensitive adhesive, whereby the cohesive force of the pressure-sensitive adhesive is enhanced and the resistance to migration into the adherend is improved.        
Patent Document 3 (Kohyo No. 9-505103) describes “a pressure-sensitive adhesive comprising a reaction product of a starting substance containing (a) from 25 to 97 parts by weight of an acrylic acid ester of monovalent alcohol, of which homopolymer has Tg of less than 0° C., (b) from 3 to 75 parts by weight of a nonpolar ethylenically unsaturated monomer, of which homopolymer has a dissolution parameter of not more than 10.50 and Tg of more than 15° C., and (c) from 0 to 5 parts by weight of a polar ethylenically unsaturated monomer, of which homopolymer has a dissolution parameter of more than 10.50 and Tg of more than 15° C., wherein the relative amounts of the acrylic acid ester, nonpolar ethylenically unsaturated monomer and polar ethylenically unsaturated monomer are selected such that the reaction product after being allowed to stand at room temperature for 72 hours has a 90° peel adhesive force of at least 2 pounds/0.5 inches for a polypropylene surface”.
Patent Document 4 (Kohyo No. 10-511126) describes “a removable pressure-sensitive adhesive comprising: (a) a pressure-sensitive adhesive particle which is a suspension polymerization product of (1) a monoolefinically unsaturated monomer having an aldehyde group or a ketone group, (2) a base monomer selected from the group consisting of an alkyl (meth)acrylate ester with the alkyl group having a carbon number of 4 to 14, a vinyl ester and a mixture thereof, and (3) an initiator soluble in oil; and (b) a polyhydrazide crosslinking agent for crosslinking the adhesive fine particles with each other”.
Patent Document 5 (Kokai No. 2007-131857) describes “a pressure-sensitive adhesive composition for surface protective film, which is a pressure-sensitive adhesive composition comprising 100 parts by weight of an acrylically copolymer containing the following components (a) to (d) and from 0.1 to 5 parts by weight of a crosslinking agent: (a) from 65 to 98.4 mass % of an acrylic acid alkyl ester with the alkyl group having a carbon number of 1 to 9, (b) from 0.5 to 5 mass % of a copolymerizable unsaturated monomer containing a hydroxyl group and/or a carboxyl group, (c) from 1 to 20 mass % of a (meth)acrylic acid alkyl ester with the alkyl group having a carbon number of 16 to 22, and (d) from 0.1 to 5 mass % of a reactive emulsifier, wherein the self-adhesive force for a stainless steel sheet 20 minutes after applying the composition at 25° C. is 50 g/inch or less, the self-adhesive force after applying and then heating the composition at 95° C. for 4 hours is 150 g/inch or less, and the surface resistance value of the glue face when coated on a polyester film substrate is 1,011Ω or less”.
Patent Document 6 (Kokai No. 8-325544) describes “a removable pressure-sensitive adhesive comprising a copolymer of a monomer mixture of A) an acrylate monomer comprising a (meth)acrylic acid alkyl ester with the alkyl group having a carbon number of 14 to 22 and being branched, B) an acrylate monomer represented by the formula: CH═CR1COOR” (wherein R1 is hydrogen or a methyl group, and R″ is a hydrocarbon or substituted hydrocarbon group having a carbon number of 4 to 8), and C) a vinyl monomer copolymerizable with the components A and B, the component A being from 30 to 90 wt %, the component B+the component C being from 70 to 10 wt %, the component B being from 60 to 0 wt %, and the component C being from 40 to 0 wt %”. Patent Document 7 describes a technique of obtaining a removable pressure-sensitive adhesive by emulsion polymerization.